This invention relates to a printing head for use in thermal printing particularly in the thermal printing of Arabic numerals on heat sensitive paper. More particularly this invention relates to a thermal print head construction which makes possible the printing of very small numerals.
In the construction of the thick film type seven bar-segment thermal print heads the possible size reduction which may be accomplished is limited to about 0.125 inches in order to provide for the necessary connections to the central areas of the seven segment figure "8" type of numeral layout. Normally the connections to the central areas of a seven segment figure "8" are made by using the multi-layer technique, that is by laying down alternate layers of printed conductors with overlaying layers of insulating material such as glass. These layers are laid down by screening techniques as required to make the necessary connections. Of course the conductors printed on one layer require holes (called "vias") in the glass laid down in the next layer to connect the conductors to conductive areas in the top layer. Those holes are of course filled with conductive material as additional layers are printed. The number of holes required for connection to a particular central area of a seven bar-segment figure "8" limits the size reduction possible for the figure. Previous thermal print heads have always required at least two connections to a central area of the figure "8" configuration and therefore the structure of the present invention allows for a reduction in size previously unattainable. Previously the overall height of the numeral could not be reduced to less than approximately 0.125 inches whereas with the present invention numerals are readily obtainable with a height of 0.084 inches.